CMOS image sensors are increasingly being used as a low cost alternative to charge coupled device (CCD) image sensors. In a CMOS image sensor, the active elements of a pixel cell perform the necessary functions of: (1) photon to charge conversion; (2) accumulation of image charge; (3) transfer of charge to the sensing node accompanied by charge amplification; (4) resetting the sensing node to a known state before the transfer of charge to it; (5) selection of a pixel for readout; and (6) output and amplification of a signal representing pixel charge from the sensing node.
CMOS image sensors of the type discussed above are generally known as discussed, for example, in Nixon et al., “256×256 CMOS Active Pixel Sensor Camera-on-a-Chip,” IEEE Journal of Solid-State Circuits, Vol. 31(12), pp. 2046-2050 (1996); and Mendis et al., “CMOS Active Pixel Image Sensors,” IEEE Transactions on Electron Devices, Vol. 41(3), pp. 452-453 (1994). See also U.S. Pat. Nos. 6,177,333 and 6,204,524, which describe the operation of conventional CMOS image sensors and are assigned to Micron Technology, Inc., the contents of which are incorporated herein by reference.
A schematic diagram of a conventional CMOS pixel cell 10 is shown in FIG. 1A. The illustrated CMOS pixel cell 10 is a four transistor (4T) cell. The CMOS pixel cell 10 generally comprises a photo-conversion device 12 for generating and collecting charge generated by light incident on the pixel cell 10, and a transfer transistor 15 for transferring the photoelectric charge from the photo-conversion device 12 to a sensing node, typically a floating diffusion region 31. The floating diffusion region 31 is electrically connected to the gate of an output source follower transistor 17. The pixel cell 10 also includes a reset transistor 16 for resetting the floating diffusion region 31 to a predetermined voltage (shown as the array pixel voltage Vaa-pix); and a row select transistor 18 for outputting a signal from the source follower transistor 17 to an output terminal in response to an address signal.
FIG. 1B is a cross-sectional view of a portion of the pixel cell 10 of FIG. 1 depicting the photo-conversion device 12. The illustrated photo-conversion device 12 is formed as a pinned photodiode. The photodiode has a p-n-p construction comprising a p-type surface layer 23 and an n-type photodiode region 22 within a p-type substrate 1. The photodiode 12 is adjacent to and partially underneath the transfer transistor 15. The reset transistor 16 is on a side of the transfer transistor 15 opposite the photodiode 12. As shown in FIG. 1B, the reset transistor 16 includes a source/drain region 32. The floating diffusion region 31 is between the transfer and reset transistors 15, 16.
A first and second dielectric layers 61 and 62 are respectively provided over the transistors 15, 16 and substrate 1. An interlayer dielectric (ILD) region 63 is provided over the second dielectric layer 62. The ILD region 63 typically includes multiple layers of interlayer dielectrics along with conductors, which form connections between devices of the pixel cell 10 and from the pixel cell 10 to other circuitry (not shown). Typically, a color filter 70, which selects a particular range of wavelengths, is provided over the ILD region 63. Over the filter 70 is a microlens 75, which focuses light onto the photodiode 12.
In the CMOS pixel cell 10 depicted in FIGS. 1A and 1B, electrons are generated by photons of electromagnetic radiation incident on the photo-conversion device 12 and are stored in the n-type photodiode region 22. These charges are transferred to the floating diffusion region 31 by the transfer transistor 15 when the transfer transistor 15 is activated. The source follower transistor 17 produces an output signal from the transferred charges. A maximum output signal is proportional to the number of electrons extracted from the n-type photodiode region 22.
It is known that not every incident photon generates an electron. The percentage of incident photons converted to electrons depends upon the quantum efficiency of the pixel cell. It is advantageous to have a pixel cell with improved quantum efficiency.